fix(relay): flush relayed connection once idle

As a relay, when forwarding data between relay-connection-source and -destination and vice versa, flush write side when read currently has no more data available.

Pull-Request: #3765.

Cargo.lock

@@ -2772,7 +2772,7 @@ dependencies = [
 
 [[package]]
 name = "libp2p-relay"
-version = "0.15.1"
+version = "0.15.2"
 dependencies = [
  "asynchronous-codec",
  "bytes",

protocols/relay/CHANGELOG.md

@@ -1,3 +1,10 @@
+## 0.15.2 - unreleased
+
+- As a relay, when forwarding data between relay-connection-source and -destination and vice versa, flush write side when read currently has no more data available.
+  See [PR 3765].
+
+[PR 3765]: https://github.com/libp2p/rust-libp2p/pull/3765
+
 ## 0.15.1
 
 - Migrate from `prost` to `quick-protobuf`. This removes `protoc` dependency. See [PR 3312].

protocols/relay/Cargo.toml

@@ -3,7 +3,7 @@ name = "libp2p-relay"
 edition = "2021"
 rust-version = "1.62.0"
 description = "Communications relaying for libp2p"
-version = "0.15.1"
+version = "0.15.2"
 authors = ["Parity Technologies <admin@parity.io>", "Max Inden <mail@max-inden.de>"]
 license = "MIT"
 repository = "https://github.com/libp2p/rust-libp2p"

protocols/relay/src/copy_future.rs

@@ -132,7 +132,14 @@ fn forward_data<S: AsyncBufRead + Unpin, D: AsyncWrite + Unpin>(
     mut dst: &mut D,
     cx: &mut Context<'_>,
 ) -> Poll<io::Result<u64>> {
-    let buffer = ready!(Pin::new(&mut src).poll_fill_buf(cx))?;
+    let buffer = match Pin::new(&mut src).poll_fill_buf(cx)? {
+        Poll::Ready(buffer) => buffer,
+        Poll::Pending => {
+            let _ = Pin::new(&mut dst).poll_flush(cx)?;
+            return Poll::Pending;
+        }
+    };
+
     if buffer.is_empty() {
         ready!(Pin::new(&mut dst).poll_flush(cx))?;
         ready!(Pin::new(&mut dst).poll_close(cx))?;
@@ -150,15 +157,17 @@ fn forward_data<S: AsyncBufRead + Unpin, D: AsyncWrite + Unpin>(
 
 #[cfg(test)]
 mod tests {
-    use super::CopyFuture;
+    use super::*;
     use futures::executor::block_on;
-    use futures::io::{AsyncRead, AsyncWrite};
+    use futures::io::{AsyncRead, AsyncWrite, BufReader, BufWriter};
     use quickcheck::QuickCheck;
     use std::io::ErrorKind;
     use std::pin::Pin;
     use std::task::{Context, Poll};
     use std::time::Duration;
 
+    #[test]
+    fn quickcheck() {
         struct Connection {
             read: Vec<u8>,
             write: Vec<u8>,
@@ -201,44 +210,6 @@ mod tests {
             }
         }
 
-    struct PendingConnection {}
-
-    impl AsyncWrite for PendingConnection {
-        fn poll_write(
-            self: std::pin::Pin<&mut Self>,
-            _cx: &mut Context<'_>,
-            _buf: &[u8],
-        ) -> Poll<std::io::Result<usize>> {
-            Poll::Pending
-        }
-
-        fn poll_flush(
-            self: std::pin::Pin<&mut Self>,
-            _cx: &mut Context<'_>,
-        ) -> Poll<std::io::Result<()>> {
-            Poll::Pending
-        }
-
-        fn poll_close(
-            self: std::pin::Pin<&mut Self>,
-            _cx: &mut Context<'_>,
-        ) -> Poll<std::io::Result<()>> {
-            Poll::Pending
-        }
-    }
-
-    impl AsyncRead for PendingConnection {
-        fn poll_read(
-            self: Pin<&mut Self>,
-            _cx: &mut Context<'_>,
-            _buf: &mut [u8],
-        ) -> Poll<std::io::Result<usize>> {
-            Poll::Pending
-        }
-    }
-
-    #[test]
-    fn quickcheck() {
         fn prop(a: Vec<u8>, b: Vec<u8>, max_circuit_bytes: u64) {
             let connection_a = Connection {
                 read: a.clone(),
@@ -275,6 +246,42 @@ mod tests {
 
     #[test]
     fn max_circuit_duration() {
+        struct PendingConnection {}
+
+        impl AsyncWrite for PendingConnection {
+            fn poll_write(
+                self: std::pin::Pin<&mut Self>,
+                _cx: &mut Context<'_>,
+                _buf: &[u8],
+            ) -> Poll<std::io::Result<usize>> {
+                Poll::Pending
+            }
+
+            fn poll_flush(
+                self: std::pin::Pin<&mut Self>,
+                _cx: &mut Context<'_>,
+            ) -> Poll<std::io::Result<()>> {
+                Poll::Pending
+            }
+
+            fn poll_close(
+                self: std::pin::Pin<&mut Self>,
+                _cx: &mut Context<'_>,
+            ) -> Poll<std::io::Result<()>> {
+                Poll::Pending
+            }
+        }
+
+        impl AsyncRead for PendingConnection {
+            fn poll_read(
+                self: Pin<&mut Self>,
+                _cx: &mut Context<'_>,
+                _buf: &mut [u8],
+            ) -> Poll<std::io::Result<usize>> {
+                Poll::Pending
+            }
+        }
+
         let copy_future = CopyFuture::new(
             PendingConnection {},
             PendingConnection {},
@@ -288,4 +295,124 @@ mod tests {
             block_on(copy_future).expect_err("Expect maximum circuit duration to be reached.");
         assert_eq!(error.kind(), ErrorKind::TimedOut);
     }
+
+    #[test]
+    fn forward_data_should_flush_on_pending_source() {
+        struct NeverEndingSource {
+            read: Vec<u8>,
+        }
+
+        impl AsyncRead for NeverEndingSource {
+            fn poll_read(
+                mut self: Pin<&mut Self>,
+                _cx: &mut Context<'_>,
+                buf: &mut [u8],
+            ) -> Poll<std::io::Result<usize>> {
+                if let Some(b) = self.read.pop() {
+                    buf[0] = b;
+                    return Poll::Ready(Ok(1));
+                }
+
+                Poll::Pending
+            }
+        }
+
+        struct RecordingDestination {
+            method_calls: Vec<Method>,
+        }
+
+        #[derive(Debug, PartialEq)]
+        enum Method {
+            Write(Vec<u8>),
+            Flush,
+            Close,
+        }
+
+        impl AsyncWrite for RecordingDestination {
+            fn poll_write(
+                mut self: std::pin::Pin<&mut Self>,
+                _cx: &mut Context<'_>,
+                buf: &[u8],
+            ) -> Poll<std::io::Result<usize>> {
+                self.method_calls.push(Method::Write(buf.to_vec()));
+                Poll::Ready(Ok(buf.len()))
+            }
+
+            fn poll_flush(
+                mut self: std::pin::Pin<&mut Self>,
+                _cx: &mut Context<'_>,
+            ) -> Poll<std::io::Result<()>> {
+                self.method_calls.push(Method::Flush);
+                Poll::Ready(Ok(()))
+            }
+
+            fn poll_close(
+                mut self: std::pin::Pin<&mut Self>,
+                _cx: &mut Context<'_>,
+            ) -> Poll<std::io::Result<()>> {
+                self.method_calls.push(Method::Close);
+                Poll::Ready(Ok(()))
+            }
+        }
+
+        // The source has two reads available, handing them out on `AsyncRead::poll_read` one by one.
+        let mut source = BufReader::new(NeverEndingSource { read: vec![1, 2] });
+
+        // The destination is wrapped by a `BufWriter` with a capacity of `3`, i.e. one larger than
+        // the available reads of the source. Without an explicit `AsyncWrite::poll_flush` the two
+        // reads would thus never make it to the destination, but instead be stuck in the buffer of
+        // the `BufWrite`.
+        let mut destination = BufWriter::with_capacity(
+            3,
+            RecordingDestination {
+                method_calls: vec![],
+            },
+        );
+
+        let mut cx = Context::from_waker(futures::task::noop_waker_ref());
+
+        assert!(
+            matches!(
+                forward_data(&mut source, &mut destination, &mut cx),
+                Poll::Ready(Ok(1)),
+            ),
+            "Expect `forward_data` to forward one read from the source to the wrapped destination."
+        );
+        assert_eq!(
+            destination.get_ref().method_calls.as_slice(), &[],
+            "Given that destination is wrapped with a `BufWrite`, the write doesn't (yet) make it to \
+            the destination. The source might have more data available, thus `forward_data` has not \
+            yet flushed.",
+        );
+
+        assert!(
+            matches!(
+                forward_data(&mut source, &mut destination, &mut cx),
+                Poll::Ready(Ok(1)),
+            ),
+            "Expect `forward_data` to forward one read from the source to the wrapped destination."
+        );
+        assert_eq!(
+            destination.get_ref().method_calls.as_slice(), &[],
+            "Given that destination is wrapped with a `BufWrite`, the write doesn't (yet) make it to \
+            the destination. The source might have more data available, thus `forward_data` has not \
+            yet flushed.",
+        );
+
+        assert!(
+            matches!(
+                forward_data(&mut source, &mut destination, &mut cx),
+                Poll::Pending,
+            ),
+            "The source has no more reads available, but does not close i.e. does not return \
+            `Poll::Ready(Ok(1))` but instead `Poll::Pending`. Thus `forward_data` returns \
+            `Poll::Pending` as well."
+        );
+        assert_eq!(
+            destination.get_ref().method_calls.as_slice(),
+            &[Method::Write(vec![2, 1]), Method::Flush],
+            "Given that source had no more reads, `forward_data` calls flush, thus instructing the \
+            `BufWriter` to flush the two buffered writes down to the destination."
+        );
+    }
 }